Problem: Let $z$ be a complex number such that $|z - 5 - i| = 5.$  Find the minimum value of
\[|z - 1 + 2i|^2 + |z - 9 - 4i|^2.\]
Answer: Let $z = x + yi,$ where $x$ and $y$ are real numbers.  Then $|x + yi - 5 - i| = |(x - 5) + (y - 1)i| = 5,$ so
\[(x - 5)^2 + (y - 1)^2 = 25.\]This simplifies to $x^2 - 10x + y^2 - 2y = -1.$

Also,
\begin{align*}
|z - 1 + 2i|^2 + |z - 9 - 4i|^2 &= |x + yi - 1 + 2i|^2 + |x + yi - 9 - 4i|^2 \\
&= |(x - 1) + (y + 2)i|^2 + |(x - 9) + (y - 4)i|^2 \\
&= (x - 1)^2 + (y + 2)^2 + (x - 9)^2 + (y - 4)^2 \\
&= 2x^2 - 20x + 2y^2 - 4y + 102 \\
&= 2(x^2 - 10x + y^2 - 2y) + 102 \\
&= 2(-1) + 102 = 100.
\end{align*}Thus, the expression is always equal to $\boxed{100}.$

Geometrically, the condition $|z - 5 - i| = 5$ states that $z$ lies on a circle centered at $5 + i$ with radius 5.

[asy]
unitsize(0.5 cm);

pair A, B, O, Z;

A = (1,-2);
B = (9,4);
O = (5,1);
Z = O + 5*dir(110);

draw(Circle(O,5));
draw(A--B);
draw(O--Z);
draw(A--Z--B);
draw(rightanglemark(A,Z,B,20));

dot("$1 - 2i$", A, SW);
dot("$9 + 4i$", B, NE);
dot("$5 + i$", O, SE);
dot("$z$", Z, NW);
[/asy]

Note that $1 - 2i$ and $9 + 4i$ are diametrically opposite on this circle.  Hence, when we join $z$ to $1 - 2i$ and $9 + 4i,$ we obtain a right angle.  Thus, the expression in the problem is equal to the square of the diameter, which is $10^2 = 100.$